1. Field of the Invention
This invention is related in general to the field of data storage systems. In particular, the invention consists of verifying that information stored in cache associated with a logical sub-system is not stale after a processing cluster is initiated.
2. Description of the Prior Art
Computer storage systems typically include one or more high-capacity disk arrays for storing digital information. These high-capacity disk arrays are often partitioned into one or more logical sub-system. A computer storage system may utilize multiple data processing servers or may use a single processing server consisting of multiple processing clusters to provide redundancy. Each data processing server or processing cluster (“cluster”) may be substantially identical and redundant. Each cluster is primarily associated with a logical sub-system, however, each cluster typically has access to the entire disk array.
It is customary for one or more hosts to send requests to clusters to read data from or write data to the logical sub-systems. The requests are delivered to the clusters over a multi-path communication device such as a bus, switch, router, bridge, point-to-point network, local area network, or other similar connecting device. Once a cluster receives a read-data request, the relevant information is retrieved from the disk array and placed into a read/write memory cache, which is relatively fast compared with other storage mechanisms. Because the cache is typically composed of volatile memory, data stored in this cache will become corrupted if the cluster loses electrical power or is taken off-line for maintenance.
Accessing data from the read/write cache is much faster than retrieving information directly from the disk array. Requested information is passed to the requesting host and a copy is maintained in the read/write cache in case it is again requested by the same or other host. If a host transmits a write request to a processing cluster, either new information is being written to the disk array or information already residing in the disk array is being modified.
In order to protect cached information, a copy of the cached information may be written to non-volatile memory. When a cluster that has been powered down or taken off-line for maintenance is re-initialized, the contents of the non-volatile memory are copied back to the volatile memory of the cache.
In a truly redundant computer storage system, each cluster may access logical sub-systems primarily associated with other clusters. In this manner, if a cluster fails, hosts may still read information from or write information to the associated logical sub-system. Accordingly, a cluster may maintain a cache for its associated logical sub-system and separate caches for each logical sub-system that it has access to. If the cluster loses power or is taken off-line, copies of each cache may be written to the non-volatile memory.
A problem may occur when a cluster is subsequently re-initialized. During the initialization process, the contents of the non-volatile memory are written back to the cluster caches. However, data in the corresponding logical sub-systems may have been modified during the time the cluster was inactive. Since multiple clusters may have cached information from the same logical sub-system, it is desirable to have a system for ensuring that each cache contain data that is coherent with each other corresponding cache and consistent with the information in the logical sub-system.